Effects of Moisture Content and Fiber Proportion on Stress Wave Velocity in Cathay Poplar (Populus cathayana) Wood

Changes in longitudinal stress wave velocity measured during the drying process of Cathay poplar (Populus cathayana) wood at different moisture contents were investigated. The test was performed at five different positions from bark to pith on each part. Five bars, cut successively from bark to pith with different fiber proportions, were also tested. The corrected velocity was calculated by dividing the velocity by the fiber proportion to negate any possible effects of wood structure on the velocity. The results showed that the longitudinal stress wave velocity decreased with increasing moisture content. Such trends were more obvious when the moisture content was lower than the fiber saturation point (FSP). The longitudinal stress wave velocity increased with increasing fiber proportion. A linear relationship between the corrected velocity and the moisture content was observed. This linear relationship was similar to the relationship between the relative velocity and the moisture content.